Generally, for lubricating oil used as a refrigerator oil, a heat pump oil, etc., various properties are required, such as (1) suitable viscosity, (2) excellent low temperature characteristics (low pour point or low flock point, etc.), (3) good compatibility with a coolant, (4) excellent high temperature stability under a coolant atmosphere, (5) excellent lubricity, (6) excellent anti-foaming properties, and the like.
On the other hand, in recent years, the tendency of increasing high efficiency, a miniaturization and weight reduction have rapidly progressed in refrigerators, heat pumps, etc., and the reciprocating system in compressors has changed to the rotary system. Further, there is a tendency that the temperature of exhaust gas is rising due to loading of an inverter or recovery of exhaust heat by a heat pump. Therefore, it is strongly required of a refrigerator oil, etc. to have high temperature stability.
In a refrigerator or a heat pump, accompanying the change in load at starting or during operation of a compressor, the pressure in a crankcase rapidly reduces so that Flon as a coolant dissolved in an oil evaporates and foams to cause various problems. That is, when foaming is marked, the frothy oil is accompanied by coolant gas (Flon) to be absorbed into a cylinder of the compressor and the inside of the cylinder is temporarily flooded with a large amount of oil. As the result, there is a fear that a valve mechanism may be damaged according to a large impact force due to the liquid hammer phenomenon. In addition, since such a large amount of oils flow out, the inside of the compressor is lacking for oil so that it offers adverse effect to lubrication at the sliding portion of metals, etc. and there is a danger of seizure occuring.
From the situation as mentioned above, there has been a desire to provide excellent anti-foaming properties to lubricating oils to be used for refrigerators or heat pumps.
The present inventor has intensively studied to solve the problems of the above conventional refrigerator oils, etc., and to develop a lubricating oil with excellent stability and improved anti-foaming properties under a Flon atmosphere.
As a result, it has been found that the above problems can be solved by blending a high viscosity polysiloxane having an epoxy structure with a base oil of a lubricating oil within a predetermined range. The present invention has completed based on such a finding.
An object of the present invention is to provide a lubricating oil composition with excellent stability under a Flon atmosphere.
Also, another object of the present invention is to provide a lubricating oil composition with excellent anti-foaming properties.